Coronal Mass Ejections

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Some of the most dramatic space weather effects occur in association with eruptions of material from the solar atmosphere into interplanetary space. These eruptions are known as coronal mass ejections, or CMEs. A large CME can contain 10.0E16 grams (a billion tons) of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path. The coronal image below shows the release of a CME at the Sun.

The event occurring here is on the side of the Sun  or the limb  which means that it will not affect us here on Earth. Sometimes, however, CMEs occur on the front side of the Sun in a location directly in front of Earth. These events appear to be very different when viewed from Earth. Instead of looking like a "bubble" of plasma, they form a circle of bright light around the Sun. This light is much dimmer than the Sun itself which is why you need to put a disk in front of the disk of the Sun in order to see what goes on around it. An example of such a "halo" event is shown in the picture below.

Near solar activity maximum, the sun produces about 3 CMEs every day, whereas near solar minimum it produces only about 1 CME every 5 days. The faster CMEs have outward speeds of up to 2000 kilometers per second, considerably greater than the normal solar wind speeds of about 400 kilometers per second. These produce large shock waves in the solar wind as they plow through it.

CMEs are sometimes associated with short periods of explosive energy release, known as solar flares. These flares frequently occur in active regions during the period around solar maximum. An example of a flare associated with an Earthward-directed CME is shown below. Flares have lifetimes ranging from hours for large gradual events down to tens of seconds for the most impulsive events. During a very strong flare, the solar ultraviolet and x-ray emissions can increase by as much as 100 times above even active-region levels. During solar maximum, approximately one such flare is observed every week. Flares heat the solar gas to tens of millions of degrees. The heated gas then radiates strongly across the whole electromagnetic spectrum from radio to gamma rays. The largest of these explosions are so bright that they can even be seen from Earth in visible light. The picture below shows the flare associated with an Earthward-directed CME event which occurred on May 12, 1997. The January 6, 1997 CME shown above did not have a flare associated with it.

Flares can accelerate protons and electrons that travel to Earth directly from the Sun along the interplanetary magnetic field (which "channels" the charged particles). These contribute to the high-energy particle environment in the vicinity of the magnetosphere if Earth's location is magnetically connected to the flaring region by the interplanetary magnetic field.

As noted above, solar flares have associated X-ray emissions  the stronger the flare, the larger the increase in the X-ray levels. The plot on the left shows a 3 day plot of 5-minute solar X-ray emission values (for the May 12, Earth-directed CME shown above) measured on the GOES 8 and 9 satellites which orbit the Earth. The letters on the right-hand y-axis are the flare classifications  i.e. an X-class flare is much more intense than the C-class flare shown here.

Major flares can be accompanied by energetic protons, which can reach Earth within 30 minutes of the flare's peak. During such an event, Earth is showered with highly energetic solar protons released from the flare site. Some of these particles spiral down Earth's magnetic field lines, reaching the upper layers of our atmosphere. These particles show up as tiny white spots in the images taken by spacecraft cameras.

The area between the Sun and the planets is called the interplanetary medium. It is often described as a vacuum, but this is not true. It is actually a turbulent area dominated by the solar wind, which flows at velocities of approximately 250-1000 km/s (about 600,000 to 2,000,000 miles per hour). Other characteristics of the solar wind (density, composition, and magnetic field strength, among others) vary with changing conditions on the Sun. In general, disturbances in the solar wind arrive at Earth 2-4 days after leaving the Sun - the CME on January 6-7, 1997 did not arrive at Earth until January 10, 1997. This CME belongs to a particular subset of CMEs, termed magnetic clouds, which usually have a greater effect on the Earth. The interplanetary space signature of a magnetic cloud is very distinct. The most easily recognized characteristics are strong magnetic fields and a large and smooth rotation of the magnetic field direction.

The figure to the left shows the magnetic field signatures for the January 6, 1997 CME. The red line shows the total magnetic field strength  and it is clear that there was a definite increase in the magnetic field strength on January 10 when the

cloud arrived at Earth. The black line shows the north-south direction of the field, and there is a very obvious smooth rotation in the direction of magnetic field.

When these disturbances arrive at Earth, they do not always have the same effect. The factor in determining how much the Earth will be effected by a CME is the direction of the magnetic field  in particular, the north-south direction, or z component. When the z component is positive, this corresponds to a northward field, which has little or no effect on the Earth. When the z component is negative, however, this corresponds to a southward field. When the interplanetary magnetic field is southward, it opposes the direction of the Earths magnetic field. In the same way that the different poles of a bar magnet attract (in contrast to like poles repelling), an interaction between the two magnetic fields will occur, allowing the energy from the solar wind to enter the Earths protective shield  the magnetosphere. It is clear from the above plot of the interplanetary magnetic field for January 9-11, 1997, that the z-component was first negative, southward, and slowly turned northward throughout the event. This means that it had an effect on the Earth  it was "geoeffective".

These solar wind disturbances can trigger global changes in Earth's magnetic field and particle populations, called magnetic storms. A magnetic storm is a period when the magnetic field measured on Earth is highly disturbed and auroras are produced. They generally last several hours to several days. The Dst  Disturbed Storm Time  Index is a measure of the magnetic field measured at Earth. An example of the Dst index associated with a storm is shown below.

This is the famous storm of March 13, 1989 which caused a transformer failure on one of the main power transmission lines in the HydroQuebec system precipitated a catastrophic collapse of the entire power grid. 6 million people lost electrical power for 9 or more hours.

A visual manifestation of a magnetic storm is the aurora. Auroras begin between 60 and 80 degrees latitude  this region is known as the auroral oval. Intensifications of auroral light are caused by increases in the numbers of electrons and ions raining or "precipitating" from the magnetosphere into the upper atmosphere in the auroral oval. As a storm intensifies, the auroras spread toward the equator. During an unusually large storm in 1909, an aurora was visible at Singapore, at the equator. The further the aurora moves towards the equator, and the brighter the emissions, the more the Earth is being effected by the CME event.

The image to the left shows an extremely large aurora that occurred March 14, 1989 at 01:51 over the southern polar regions. The picture to the right is a re-mapping of that image into northern hemisphere to give an idea of what the auroral oval would have looked like if the spacecraft had been over the northern hemisphere at this time. From this image you can see that you would have been able to see the aurora in Texas and Florida.<